Controlled-source electromagnetic (“CSEM”) surveys are an important geophysical tool for evaluating the presence of hydrocarbon-bearing strata within the earth. CSEM surveys typically record the electromagnetic signal induced in the earth by a source (transmitter) and measured at one or more receivers. The behavior of this signal as a function of transmitter location, frequency, and separation (offset) between transmitter and receiver can be diagnostic of rock properties associated with the presence or absence of hydrocarbons. Specifically, CSEM measurements are used to determine the spatially-varying resistivity of the subsurface.
In the marine environment, CSEM data are typically acquired by towing an horizontal electric dipole transmitting antenna 10 among a number of autonomous receivers 11 positioned on the seafloor 12 (FIG. 1). The receivers have multiple sensors designed to record different vector components of the electric and/or magnetic fields. The transmitter is typically towed 10-50 m above the seafloor. Alternative configurations include stationary transmitters on the seafloor (Constable, “System and Method for Hydrocarbon Reservoir Monitoring Using Controlled-source Electromagnetic Fields,” WO 2004/053528A1) as well as magnetic transmitter antennae and vertical transmitters (Fielding and Lu, PCT patent application publication No. WO 2005/081719, “System and Method for Towing Subsea Vertical Antenna”; MacGregor et al., “Electromagnetic Surveying for Hydrocarbon Reservoirs,” PCT patent publication No. WO 2004/008183). The transmitting and receiving systems typically operate independently (without any connection), so that receiver data must be synchronized with shipboard measurements of transmitter position and with the measured transmitter current waveform by comparing clock times on the receivers to time from a shipboard or GPS (Global Positioning System) standard.
CSEM data are typically interpreted in the temporal frequency domain, each signal representing the response of the earth to electromagnetic energy at that temporal frequency. Temporal frequency domain means the data is transformed, typically by Fourier transformation, such that the dependence of the data on time becomes dependence on frequency. In raw data, the strength of each frequency component varies depending on how much energy the transmitter broadcasts (i.e., the amplitude of each component in the transmitter's frequency spectrum) and on the receiver sensitivity at that frequency. These transmitter and receiver effects are typically removed from the data prior to interpretation. FIGS. 2A-B depict raw receiver data 21 together with the transmitter waveform 22 that gave rise to it. FIG. 2A displays measured data on a time scale of several hours while FIG. 2B shows the received signal (and, for reference, the transmitted signal) on a much shorter time scale, comparable to the transmitter signal period, typically between 4 and 32 seconds. (The vertical scale applies only to the receiver signal.)
In practice, the receiver data are converted to temporal frequency by dividing (or “binning”) the recorded time-domain data into time intervals (x1, x2, and x3 in FIG. 3A) equal to the transmitter waveform period (FIG. 3A) and determining the spectrum within each bin by standard methods based on the Fourier Transform (FIG. 3B). (The phases of the spectral components are not shown.) With each bin is associated a time, typically the Julian date at the center of the bin. Since the transmitter location is known as a function of time, these bins may be interchangeably labeled in several different ways: by Julian date of the bin center; by transmitter position; by the signed offset distance between source and receiver; or, by the cumulative distance traveled by the transmitter relative to some arbitrarily chosen starting point.
In general, the received signals are made up of components both in-phase and out-of-phase with the transmitter signal. The signals are therefore conveniently represented as complex numbers in either rectangular (real-imaginary) or polar (amplitude-phase) form.
More details of a typical marine CSEM transmitter are shown in FIG. 4. In addition to providing the forces needed to deploy and tow the transmitter, the tow cable 43 supplies power to the transmitter from the ship. That power is typically supplied at high voltage and low current to reduce ohmic losses in the tow cable, which may be several kilometers in length. The electric field generated in the earth is proportional to the transmitted current, so the transmitter 40 includes a step-down transformer to supply low-voltage, high-current power to the antenna electrodes 41 and 42. Taken together, the antenna electrodes and the wires connecting them form an electric dipole transmitting antenna, typically in the range of 100 to 300 meters in length. In typical operation, the antenna injects 800-1000 amperes of current at 100 Volts or less.
The transmitter signal may be a more complex waveform than the square wave depicted in FIGS. 2 and 3. For example, the tripeak waveform shown in FIGS. 5 and 6 is designed to generate roughly equal amplitudes for the fundamental frequency, the second harmonic, and the fourth harmonic (X. Lu and L. J. Srnka, “Logarithmic Spectrum Transmitter Waveform for Controlled-Source Electromagnetic Surveying,” PCT Patent Application Publication No. WO/2005/117326). The switching times referenced in FIG. 5 for the tripeak waveform are as follows, in fractions of the period T: T0=0; T1= 18/256; T2= 60/256; T3= 67/256; T4= 110/256; T5= 147/256; T6= 186/256; T7= 198/256; T8= 237/256; and T9=1. The waveform is typically repeated after the indicated period, T, resulting in a fundamental frequency 1/T. The switching times, T1, T2, etc., are conveniently chosen for a transmitter operating at a 256 Hz carrier frequency. The amplitude depends on the amount of current the transmitter can deliver. FIG. 6 shows the frequency domain amplitudes of the tripeak waveform shown in FIG. 5. Most of the source energy appears at frequencies 1/T, 2/T, and 4/T Hz. Other peaks (7/T, 10/T, 14/T, . . . ) also contain useful amounts of signal. The very small peaks (3/T, 5/T, 8/T, . . . ) do not typically generate a usable response in the earth. By contrast, a square waveform has no amplitude at the even harmonics while the amplitudes of its odd harmonies are proportional to 1/N, where N is the number of the harmonic.
The transmitter described in FIG. 4 may include electronic components to rectify or polarity-flip the sinusoidal current supplied from the ship to produce waveforms having the desired frequency and amplitude characteristics. Waveform-generation techniques are described, for example, in Chave, et al, “Electrical exploration methods for the seafloor,” in Electromagnetic Methods in Applied Geophysics, Volume 2, M. Nabighian (ed), Soc. Explor. Geophys., Tulsa, 931-966 (1991); Cox, et al, “Controlled source electromagnetic sounding of the oceanic lithosphere,” Nature, 320, 52-54 (1986); and Sinha and MacGregor, PCT patent publication WO 03/034096A1. In general, these techniques operate by variously withholding current from the transmitter antenna, directing a half-cycle of the input to the antenna, or directing a half-cycle of the input to the antenna with reversed electrical polarity. This method is very effective at approximating low-frequency waveforms (for example, 32 Hz and below) from inputs with a relatively high carrier frequency (for example, 256 Hz and above). As an aid to understanding the method, FIG. 7 shows how a 0.5 Hz tripeak waveform might be composed from input current at a carrier frequency of 16 Hz. The 16 Hz carrier is for illustration only; a much higher frequency is typically used. In practice, capacitive effects in the transmitter smooth out the high-frequency sinusoidal ripples to some degree.
Among the problems currently hampering CSEM surveying are the following:                lowering the cost of CSEM surveys by reducing the time spent operating sources and receivers;        minimizing the time spent acquiring data in the field to overcome constraints imposed, for example, by acceptable weather, fishing seasons, or vessel traffic around production facilities (CSEM surveys are typically, but not necessarily, conducted in a marine environment);        improving the quality (signal-to-noise ratio) of CSEM data by increasing the total electromagnetic signal injected into the earth; and,        improving the resolution of CSEM data by increasing the range of spatial and temporal frequencies that can be effectively transmitted into the earth.        
Some current measures to mitigate these problems are discussed in the following paragraphs.
The leasing and operation of the survey vessel is a significant part of the total cost of marine CSEM surveys. While the exact percentage will vary from survey to survey, the time spent operating the transmitter can easily account for more than half of the survey cost. To transmit as much signal as possible into the subsea sediments, the antennae is typically towed at an elevation of 50 meters or less above the seafloor. To navigate the antenna safely and effectively at this elevation while maintaining acceptable spatial resolution, it must be towed at relatively low speeds—typically 2 knots (≈1 m/s) or less—so that long tow lines can require up to a full day to carry out.
The total electromagnetic signal injected into the earth is a key factor in determining the size and depth of hydrocarbon accumulations that may be identified using CSEM data. Noise levels measured by the receivers will vary from survey to survey, and some data processing methods are available to help decrease this noise, but the ability to detect the earth's response to injected signals is ultimately limited by this noise floor. The most direct way to boost signal up above noise is to increase the dipole moment of the transmitter (injected current times antenna length). The antenna length is constrained by the capabilities of the launch and recovery equipment on the vessel and the need to keep the antenna neutrally buoyant. A more direct method to increase the dipole moment is to increase the injected transmitter current.
As is well known from the theory of Fourier Analysis and skin depth considerations, the ability to resolve individual geologic features with a CSEM survey is enhanced by the addition of more temporal frequencies to the transmitter waveform and by occupying more spatial locations with the transmitter. The range of temporal frequencies in use is known as the source bandwidth.
As previously discussed, Lu and Srnka custom-designed the transmitter waveform in order to more efficiently spread the available transmitter current among the most important frequencies. Their tripeak waveform is a sequence of transmitter waveforms that balances the current amplitude at three chosen frequencies.
Processing methods, such as subtracting noise estimated at non-transmitted frequencies (Willen, “Estimating Noise at One Frequency by Sampling Noise at Other Frequencies,” PCT international patent application PCT/US06/01555, filed on Jan. 17, 2006) and stacking have been used to mitigate noise in CSEM data.
Workers in the field of marine seismic exploration have made use of multiple seismic sources towed from a single vessel and of sources towed from multiple vessels. See, for example, FIG. 4 of U.S. Pat. No. 5,924,049 to Beasley et al., “Methods for Acquiring and Processing Seismic Data.” The immediate impact of using multiple sources is to achieve better spatial resolution of the subsurface by occupying a broader distribution of source locations without significantly increasing the time spent acquiring data. Beasley et al. further disclose a method of energizing more than one source at the same time in order to minimize the cost of the additional spatial resolution. Their method is to reconstruct the data that would have been acquired had the sources been energized separately in time. They disclose methods of reconstructing such data based on the “dip” of seismic events (the slope of seismic arrivals functions of the offset between source and receiver) from different sources. See also UK Patent Application GB 2,411,006 filed Feb. 16, 2004, naming inventors MacGregor, et al., titled “Electromagnetic Surveying for Hydrocarbon Reservoirs.”
In the field of land seismic acquisition, U.S. Pat. No. 4,823,326 to Ward groups vibrator sweep signals into sets of four or more sweeps and introduces a phase factor to be applied to each sweep in the set. By appropriately selecting these phase factors, Ward can arrange to recover the data that would have been acquired separately by two or more vibrator sources from data collected while the sources were operated simultaneously. Ward's technique involves correlating vibrator data with pilot signals, which produces time-domain seismic data.
A series of patents to Allen and others address the problem of separating the seismic responses (acoustic waves) of two or more simultaneously operating vibratory sources. U.S. Pat. No. 5,822,269 discloses a method for separating and pre-processing vibratory source data by varying the phase of the vibratory sources according to two patterns. U.S. Pat. No. 5,715,213 discloses a method for recording and pre-processing high fidelity vibratory seismic data that includes the steps of measuring the motion of the vibrator which is related to the vibrator applied force times a transfer function of minimum phase, causal, linear system relating the actual vibrator output with the measured vibrator motion, and separation of signals according to generating source. U.S. Pat. No. 5,721,710 discloses a method of separating the effects of the earth response on vibratory energy from individual ones of multiple vibrators as detected by geophones in the course of a seismic survey.
While the objectives of any particular survey and the conditions encountered near the seafloor may provide some flexibility, there are limits to what can be done toward saving time by towing the transmitter more rapidly, since faster tow speeds make it more difficult to control the transmitter's elevation above the seafloor.
Considerable power-generation capacity can be made available on the tow ship, but the overall current available to the antenna is limited by physical size of the tow cable. Using a larger tow cable would mean using larger hoists to deploy the cable and larger winches to direct the transmitter motion through the heavier cable. An even more serious problem would be keeping a larger tow cable cool enough to avoid damage while on its take-up reel.
Processing methods such as stacking address the issue of increasing signal-to-noise ratio but only impact the survey cost or resolution in so far as these criteria both depend on signal-to-noise ratio. In general, data acquisition techniques that address the above-identified problems can be practiced together with a variety of processing techniques that increase signal-to-noise ratio.
In order to reconstruct the data that would have been acquired by separate (non-simultaneous) source excitations, Beasley et al. must have a significant physical separation of the sources such as positioning a source at either end of a marine streamer containing the receivers. Only by having this physical separation and relatively high bandwidth (compared to CSEM surveys) can they establish the offset versus time trajectories (their FIGS. 12 through 18) needed to separate the data from each source by means of multi-channel, f-k, or Radon filtering techniques. They must employ a filtering technique that is sensitive to these trajectories.
Ward recognizes the opportunity to phase-encode separate vibrator sweeps within a time-sequential set of sweeps. However, vibrator sweeps have a continuous frequency spectrum quite unlike the discrete CSEM source spectra typified by FIG. 6. As a result, vibrator sweep sets must include a minimum of four sweeps in order to separate the data from two vibrators. Still larger sweep sets are required in order to employ more vibrators.